Frequently, clocked systems are used for regulating currents, such as in electric motors or magnetic valves. These systems consist generally of a coil, a power switch, such as a power transistor, and a freewheeling diode, which protects the arrangement from voltage peaks that can occur due to the self-induction of the used coils. Here, the measurement of the coil current represents a large problem. This current measurement is mostly performed via an external or internal shunt resistor. A shunt resistor is a high-precision ohmic resistor, which is connected in series with the arrangement to be measured. In order to obtain a measure for the current through an array, the voltage drop at the shunt resistor is measured. Thus, the voltage measured at the shunt resistor is a direct measure for the current through the whole arrangement. For keeping the power dissipation caused in the shunt resistor as low as possible, the shunt resistances are generally low and lie, for example, between 50 and 200 mOhm. As a consequence, the voltage drop measured at the shunt resistor is also relatively low. The main problems when designing a measurement amplifier are the low voltage drop at the shunt and the high common mode jumps at the input at the amplifier, which can be caused by the self-induction, for example of an electric motor. For example, negative voltages occur at an upstream shunt resistor in the case of a free-running electric motor, wherein in the normal operating case the shunt resistor comes to lie on the battery potential or the supply voltage potential, respectively. In order to compensate these possibly high potential voltages of the voltage difference that can be measured at the shunt resistor, a so-called level shifter is used. A level shifter is an arrangement of resistive elements and current sources, which allows shifting a voltage difference from a first potential to a second potential.
FIG. 8 exemplarily shows such an ideal level shifter. The voltage Ushunt 810 is to be measured at the shunt resistor 800 with the resistance Rs. Therefore, this voltage is tapped at the shunt resistor 800 and converted to a desired voltage potential via two resistors 820 and 830, each having a resistance of Rcm, which are fed via two current sources 840 and 850. The voltage at the shunt resistor 800 is then tapped at the current sources 840 and 850 as Ushunt 860. In order to not mismatch the voltage Ushunt 810, which drops at the shunt resistor 800, it is required that the two resistors 820 and 830 have identical resistances Rs. Since actual resistors always have a tolerance, an absolute equality between the resistors 820 and 830 can practically not be obtained.
FIG. 9 shows a further circuit of a level shifter, wherein the resistor difference between the two resistors 820 and 830 is considered as additional resistor 900 with the resistance dR. The resistor difference dR has the consequence that the voltage difference 910, that can now be measured between the two current sources 840 and 850 no longer corresponds to the value Ushunt 810, but is mismatched by an additional voltage drop at dR, and thus the voltage Ushunt+dR·Ishift is measured. Thus, the represented level shifter, which is, for example, used to adjust a steady component of the shunt voltage from the battery potential to an input region of an exact preamplifier, has a mismatch. The accuracy of the circuit is determined by the deviation of the current sources or the resistors, respectively. By trimmed or choppered, respectively, current sources, the deviation of the shift currents then can be reduced, but the resistor deviation remains. For example, for shifting the shunt voltage (which is approximately 200 mV) from Vbat=14 V to Vinput=4 V, a current of 1 mA and a resistor of 10 kOhm are required. If a resistor tolerance of 0.5% is assumed, this corresponds to 50 Ohm and a voltage mismatch of 50 mV (50 mV=50 Ohm·1 mA). Thus, the error is 50 mV at a maximum signal voltage of 200 mV. For avoiding this problem, high-precision resistors are used, which are represented as 820 and 830 in FIGS. 8 and 9. So-called laser trimming obtains high-precision trimming of ohmic resistors, which is expensive and cost-intensive.